Perpendicular magnetic anisotropy and its voltage control in MgO/CoFeB/Mo/CoFeB/MgO junctions

We study the perpendicular magnetic anisotropy (PMA) in the MgO/CoFeB (CFB)/MgO junctions with an angstrom-thick Mo spacer layer separating the CFB layer. Perpendicularly magnetized CFB/Mo/CFB films are achieved for a wide range of CFB thicknesses, and a large PMA energy density of >0.3 mJ m(-2) is demonstrated by tuning the thickness ratio of the two CFB layers as well as the thickness of the Mo spacer layer. The PMA in the MgO/CFB/Mo/CFB/MgO is controlled by a voltage applied across the junction, and a sign inversion in the voltage-controlled magnetic anisotropy effect is clearly observed between the 'top free' and 'bottom free' magnetic tunnel junctions, in which the CFB/Mo/CFB layers are fabricated on top and bottom of an MgO barrier layer, respectively. Nanostructural analyses reveal the difference in the morphology of the top free and bottom free magnetic tunnel junctions and also suggest that the flatness of CFB/MgO interface is rather important for improving the efficiency of the voltage-controlled magnetic anisotropy.